Hosts may be expected to connect to the network via multiple links in active-active mode (as opposed to active-standby) for load sharing and increased bandwidth. These links are often seen as part of a layer-2 ether-channel (link aggregation, IEEE 802.3ad). Using existing virtual Port-Channel (“vPC”) technology, these links may terminate on two different physical devices. However, when bridging or routing an IP multicast packet, the host may only expect a single copy of a multicast from the network on the Port-Channel (“PC”) or a vPC.
It may be common to employ an encapsulation, also known as an overlay network, within data center networks to transport multi-destination frames in an outer delivery tree. Use of a delivery tree, based on an overlay multicast group may allow inner multicast groups belonging to different layer-2 segments to share a mapped delivery tree identified by the outer header encapsulation.
When multicast recipients happen to be in different VLANs, a router may implement a function of replicating multicast traffic to one or more VLANs dependant upon hosts registering their interest in specific multicast groups within respective VLANs. In a layer-2 overlay fabric topology, this may mean that a packet needed routing with result in multiple copies (one in each VLAN) of multicast traffic sent to the fabric, which is inefficient because of replication across VLANs. An easy way to avoid the multiple copy problem is to use a routed segment that just interconnects the replication points in the network. However, such an approach may result in the loss of the originating VLAN, which may be needed to avoid sending frames back to the vPC link in the same VLAN. This may force the network to replicate the multicast traffic, which consumes precious bandwidth.